Ultrasonic Flow Meter VS Electromagnetic Flow Meter

Electromagnetic flow meters only work when the substance they measure is conductive (liquids, like water). They work according to Faraday’s law of induction. The flow meter creates a magnetic field, and the liquid that passes through the meter contains positive and negative ions. The magnetic field will separate these + and – ions, causing a voltage difference to appear in the liquid. The quicker the liquid flows, the more electricity (voltage) will be generated.

Electromagnetic flow meters are usually applied to installations that are meant for measuring water or waterbased (chemical) liquids.

What works: drinking water, rainwater, groundwater, waterbased solutions, acids, alkaline solutions and more.

What doesn’t work: distilled water (because it is not conductive), fossil oils (hydrocarbons), and gases.

Electromagnetic Flow Meter typical locations:

• Influent flow meter
• Effluent flow meter
• Sludge return flow meter
• Aeration tank circulation flow
• Chemical dosing pipelines

An ultrasonic flow meter is another kind of non-obstructing flow meter that can measure liquids with a limited viscosity. The way in which flow is measured however differs from the electromagnetic flow meter. Ultrasonic flow meters work by sending out ultrasonic (sound) pulses. The meter contains at least 2 sound transmitter-receivers. The speed of the measured medium is determined by the difference of the transit time at which the pulse travels between the transmitters and the receivers.

What works: clean fluids (no solids), gases and fossil oils (hydrocarbons).

What doesn’t work: ultrasonic flow meters are sensitive to solids (more than 5 to 10%) or bubbles.

Ultrasonic flow meters are mainly used for:

• Large water transmission pipelines
• Existing plants upgrading
• Temporary measurement
• Difficult installation locations

What is the difference between electromagnetic and ultrasonic flow meters?

The biggest difference between ultrasonic and electromagnetic flow meters is that ultrasonic flow meters do not require a conductive liquid to work. They also measure gases but fare less well with liquids that contain many solids or bubbles. Ultrasonic meters are able to withstand high temperatures and high pressure. Along with being able to withstand aggressive fluids (in case of clamp-on ultrasonic flowmeters).

The biggest difference between ultrasonic and electromagnetic flow meters is how they measure (and consequently what they can measure). Electromagnetism works based on whether a liquid is conductive and flow is measured based on electricity. Ultrasonics work based on sound pulses and measure the time between emitting and receiving a pulse. Ultrasonic flow meters can measure gases, liquids and vapours, while electromagnetic flow meters work solely on (conductive) liquids.

Last but not least, it’s worth noting that electromagnetic flow meters are often cheaper than ultrasonic flow meters.

Basic Working Principle

Item	Ultrasonic Flow Meter	Electromagnetic Flow Meter
Measurement principle	Measures the velocity of fluid using ultrasonic sound waves	Uses Faraday’s Law of electromagnetic induction
How it works	Calculates flow based on the difference in ultrasonic signal travel time upstream/downstream	Conductive liquid moving through a magnetic field generates voltage proportional to velocity
Contact with liquid	Usually no contact (clamp-on type)	Electrodes contact the liquid

Installation

Ultrasonic Flow Meter Installation

Advantages

• Can be installed without cutting the pipe (clamp-on type)
• No shutdown required
• Easy retrofit on existing pipelines
• Suitable for temporary measurement

Disadvantages

• Requires accurate pipe parameters:
  • Pipe material
  • Pipe diameter
  • Wall thickness
  • Fluid temperature
• Installation position is critical
• Needs straight pipe length for good accuracy

Electromagnetic Flow Meter Installation

Advantages

• Very stable measurement
• Less affected by pipe conditions
• High accuracy

Disadvantages

• Must be installed inline
• Requires pipe cutting and flange connection
• Installation requires system shutdown

Accuracy Comparison

Parameter	Ultrasonic Flow Meter	Electromagnetic Flow Meter
Typical accuracy	±1%~±2%	±0.2%~±0.5%
Repeatability	Good	Excellent
Long-term stability	Medium	Excellent
Calibration requirement	Higher	Lower

For billing, custody transfer, and strict process control, electromagnetic flow meters usually perform better.

Fluid Compatibility

Electromagnetic Flow Meter Fluid Compatibility

Requires: Electrically conductive liquids.

Typical minimum conductivity:

• Water: OK
• Wastewater: Excellent
• Sludge: Excellent
• Chemical solutions: Depends

Applications:

• Municipal wastewater
• Industrial wastewater
• Drinking water
• Sludge pipelines
• Chemical dosing systems

Cannot measure:

• Pure water with very low conductivity
• Oil
• Gas
• Steam

Recommended: Electromagnetic Flow Meter

Why:

Sludge flow measurement

Recommended: Electromagnetic Flow Meter

Reasons:

• Sludge contains solids
• Conductivity is high
• Stable measurement required

Large diameter water pipelines

Recommended: Ultrasonic Flow Meter

Reasons:

• Installing a large electromagnetic meter is expensive
• No need to cut huge pipes
• Clamp-on installation is convenient

Temporary flow testing

Recommended: Ultrasonic Clamp-on

Example:

• Check pump performance
• Verify existing flow meters
• Measure unknown pipelines

Cost Comparison

Cost Factor	Ultrasonic Flow Meter	Electromagnetic Flow Meter
Equipment cost	Medium	Medium-high
Installation cost	Low	Higher
Maintenance cost	Very low	Very low
Pipe modification	No	Yes
Large diameter application	Very economical	Expensive

Maintenance Comparison

Item	Ultrasonic	Electromagnetic
Sensor cleaning	Not required (clamp-on)	Sometimes electrode cleaning
Moving parts	None	None
Fouling problems	Low	Medium
Lifetime	10–15 years	10–20 years

Which One Should You Choose?

Choose Electromagnetic Flow Meter if:

• Measuring wastewater
• Measuring sludge
• Permanent installation
• High accuracy required
• Pipe size DN25–DN600 commonly
• Conductive liquid

Most wastewater treatment plants use electromagnetic flow meters.

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Choose Ultrasonic Flow Meter if:

• Existing pipeline cannot stop operation
• Large diameter pipe (>DN600)
• Temporary measurement
• Non-conductive liquid
• Need low installation cost

Simple Selection Guide

Application	Recommended Meter
Municipal wastewater inlet	⭐ Electromagnetic
Activated sludge flow	⭐ Electromagnetic
Return activated sludge (RAS)	⭐ Electromagnetic
Chemical dosing water	Electromagnetic / Ultrasonic
Raw water pipeline	Both
DN1000+ pipeline	⭐ Ultrasonic
Pump testing	⭐ Ultrasonic
Cooling water	Ultrasonic
Oil	Ultrasonic
Pure water	Ultrasonic